The airports in Miami and Fort Lauderdale were hit the hardest, cancelling 573 flights -- the vast majority of the 654 U.S. flights grounded overall because of the storm as of Sunday morning, according to the flight-tracking service FlightAware.

American Airlines and its American Eagle affiliate cancelled 460 flights. The last American flight left Miami at noon Sunday. The airline hopes that inbound and outbound Miami flights can resume by 10 a.m. Eastern time Monday, said airline spokeswoman Andrea Huguely.

Isaac, already carrying winds of more than 60 miles an hour, was expected to cross the Keys by late afternoon.

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